The present invention is related to the field of partial response maximum likelihood detection trellis based channels.
Traditional peak detection channels in magnetic disk drives have been replaced by partial response maximum likelihood (PRML) read channels to allow for higher recording densities where individual symbols written within bit cells on the magnetic disk may overlap neighboring bit cells resulting in inter-symbol interference. PRML read channels make use of the read signal inside a desired bit cell and its neighboring bit cells to decide the desired bit cell""s content. Information from the neighboring bit cells results in improved bit error rates over conventional peak detection channels in the presence of additive noise.
PRML read channels use a complicated set of equations to make an optimal bit decision based upon the read signal within several neighboring bit cells. PRML read channels became practical when Andrew J. Viterbi developed an algorithm that significantly reduced the number of computational steps required to make the optimal bit decision. Viterbi""s algorithm is generally called a xe2x80x9ctrellisxe2x80x9d and is represented by a two-dimensional graph of various allowable data patterns.
Trellis-based PRML read channels are not ideal in that they are more susceptible than peak detection channels to variations in the read signal""s monopulse shape, amplitude, and timing. A PRML read channel will produce higher bit error rates than a peak detection read channel when the channel variations are significantly large. For this reason, PRML read channels were adopted in disk drives before tape drives. Tape drives have inherent channel variations that reduced the potential advantages of PRML read channels.
Attempts have been made to develop various adaptive algorithms that adjust PRML read channels in real time to account for read channel variations. However, these adaptive algorithms have only been able to adapt to slowly-varying parameters. The read channels parameters in tape drive applications can vary within a small number of bit cells. The time lag of the adaptive algorithms generally cause them to provide little benefit in tape applications. In some cases the adaptive algorithms cause increased bit error rates. What is desired is an approach that reduces the sensitivity of standard Viterbi trellis read channels to channel parameter variations.
The present invention is an improved maximum likelihood detector with a trellis having three or more dimensions. Trellis dimensions include a state dimension and a sample dimension found in a conventional Viterbi trellis. Additional dimensions are added to the trellis for adaptation to additional parameters that are responsive to channel variations. Consequently, the improved maximum likelihood detector provides lower error rates than conventional Viterbi trellis read channels in magnetic tape applications where channel variations can be significant.
Additional parameters include, but are not limited to, a pulse shape parameter, a pulse spacing parameter, a pulse amplitude parameter, a pulse time parameter, a media velocity parameter, and a media tension parameter. These parameters may vary in some magnetic tape applications at rates faster than some conventional adaptation algorithms can handle.
Rules are established to control movement along the additional dimensions. In the preferred embodiment, a path through the trellis can move up to a predetermined maximum number of unit lengths along any additional dimension for every one unit length movement along the sample dimension. Once a movement has been made along a respective additional dimension from a first value to a second value then the path must dwell at that second value for a minimum number of samples, preferably at least two samples.
Accordingly, it is an object of the present invention to provide an improved maximum likelihood detector having a three or more dimensional trellis, and rules for establishing paths through the trellis dimensions.
These and other objects, features and advantages will be readily apparent upon consideration of the following detailed description in conjunction with the accompanying drawings.